The primary objective is to determine the relationship between clinical growth in pubertal girls and boys and shifts in the irregularity of underlying growth hormone (GH) secretory patterns. Novel clarifications of this time-evolution could be significant in determining the proper timing of hormone replacement therapies in pathophysiological settings such as Turner's Syndrome or constitutional delay of growth and adolescence, and in enhancing physiologic understanding. We will also investigate gender differences in the growth-secretory pattern relationship. (Secretory) irregularity will be quantified by approximate entropy (ApEn), developed by the principal investigator. ApEn has been widely applied scientifically, with numerous applications to hormonal data providing complementarity to pulse detection methods. In two (cross-sectional; longitudinal) recent 6 year peripubertal studies of boys, we observed a highly significant correlation between increased OH secretory irregularity and subsequent growth velocity increase 4-8 months later, providing predictive capability. Phase I focuses on thematically similar data analysis from an improved longitudinal design, including girls and boys, additional (body composition) measurements, and sharper assays. Phase II will incorporate sex-steroid hormone and anthropometric data into regression models to obtain parsimonious factors most predictive of ensuing growth, applying this design to the aforementioned pathophysiologies, and to healthy children. PROPOSED COMMERCIAL APPLICATIONS: Software development of an ApEn program for application to general endocrine hormone secretion time series; incorporation of ApEn software into existing hormonal administration systems. In particular, collaborations with pharmaceutical companies who manufacture growth hormone.